A combination Wham Shot and Wham Line from the end of the two-part premiere reveals that Della Duck, Donald's sister and the mother of Huey, Dewey and Louie is going to be important in this series. The final scene of season 1 even reveals that Della is alive. In all other media involving the triplets, their mother is absent and barely mentioned in dialogue or flashbacks. So Della becoming a major character in the second half of season 2 and the following season 3 was a bit of a shock to fans familiar with the Disney Ducks Comic Universe.
The Season 1 episode "Sky Pirates In The Sky!" has Don Karnage and his crew of Sky Pirates from the originally unrelated show TaleSpin as the villains.
Season 2 features both Donald's cousin Fethry Duck and his Neighbor Jones, reimagined as an anger management therapist, two characters from the Disney Ducks Comic Universe. They are far more well-known and popular outside of the U.S. in Europe and Brazil, so their making their debut in animation was a welcome surprise for many comic book fans.
Season 2 also introduces popular characters José Carioca and Panchito Pistoles from The Three Caballeros to the franchise, taking on the role of Donald's old college friends, with "The Three Caballeros" being the name of their band.
While Goofy's appearance was revealed by the season 3 poster (see above), many wondered if this would mean his son Max Goof might appear as well. He does, but is only shown in pictures - the pictures surprisingly also feature Max' best friend P.J. and his movie love interest Roxanne.
The series finale revealed that Manny the Headless Man-Horse was in factthis continuity's counterpart to Goliath, of all beings, complete with Keith David voicing him again! This was a complete curveball, given that while Gargoyles was a part of the Disney Afternoon, the show's Darker and Edgier tone and worldbuilding of its own — including humans — definitely couldn't be melded into the continuity like with the other shows.
Ludwig von Drake was only portrayed in flashbacks and with his heirs making an appearance in season 2, many jumped to the conclusion that this von Drake is a Posthumous Character, but he actually shows up in the series finale, still alive.